This invention relates to optical communications and specifically to a Raman amplifier and a pump assembly for the Raman amplifier.
Wave division multiplexing (WDM) increases bandwidth in optical communications by providing for communication over several wavelengths or channels. For long haul optical communications the optical signal must be periodically amplified. To maximize WDM capacity, it is desirable that the optical bandwidth of the system be as wide as possible. Raman amplification is one of the amplification schemes that can provide a broad and relatively flat gain profile over the wavelength range used in WDM optical communications. (See Y. Emori, xe2x80x9c100 nm bandwidth flat-gain Raman Amplifiers pumped and gain-equalized by 12-wavelength channel WDM Diode Unit,xe2x80x9d Electronic Lett., Vol. 35, no 16, p. 1355 (1999) and F. Koch et. al., xe2x80x9cBroadband gain flattended Raman Amplifiers to extend to the third telecommunication window,xe2x80x9d OFC""2000, Paper FF3, (2000)). Raman amplifiers may be either distributed or discrete (See High Sensitivity 1.3 xcexcm Optically Pre-Amplified Receiver Using Raman Amplification,xe2x80x9d Electronic Letters, vol. 32, no. 23, p. 2164 (1996)). The Raman gain material in distributed Raman amplifiers is the transmission optical fiber, while a special spooled gain fiber is typically used in discrete Raman amplifiers.
Raman amplifiers use stimulated Raman scattering to amplify a signal at a signal wavelength. In stimulated Raman scattering, radiation power from a pump radiation source is transferred to an optical signal to power from a pump radiation source is transferred to an optical signal to increase the power of the optical signal. The frequency (and therefore photon energy) of the radiation emitted by the pump radiation source is greater than the frequency of the radiation of the optical signal. This down shift in frequency from the pump frequency to the signal radiation frequency is due to the pump light interaction with optical phonons (vibrations) of the Raman gain material, i.e., the medium through which the pump radiation and the optical signal are traversing.
The Raman gain material in Raman amplifiers can be the transmission optical fiber itself. The Raman gain coefficient for a silica glass fiber (such as are typically used in optical communications) is shown in FIG. 1 as a function of the wavelength shift relative to a pump wavelength of around about 1400 nm. As can be seen, the largest gain occurs at about a 100 nm shift. Thus, the maximum gain for a single pump wavelength of about 1400 nm will occur at a signal wavelength of about 1500 nm. Since the optical gain is proportional to the pump intensity, the gain of the signal of a Raman amplifier is the product of the Raman gain coefficient and the pump intensity.
The gain profile having a typical bandwidth of 20-30 nm for a single pump wavelength is too narrow for WDM optical communications applications where a broad range of wavelengths must be amplified. To broaden the gain profile, Raman amplifiers employing multiple pump wavelengths over a broad wavelength range have been suggested for use in WDM optical communication applications. For example, it has been suggested to use twelve pump wavelengths to achieve a 100 nm bandwidth Raman amplifier.
In order for a flat gain profile to be achieved, the pumpxe2x80x94pump interactions generally require that the shorter pump wavelengths have a higher pump power than the longer pump wavelengths. This is so because energy from the shorter wavelength (higher photon energy) pumps is transferred to the longer wavelength pumps due to stimulated Raman scattering. To compensate for the pumpxe2x80x94pump energy loss at shorter wavelengths, the shorter pump wavelengths should have increased power.
A typical pump power-pump wavelength scheme to achieve a relatively flat and broad Raman gain profile is illustrated in FIG. 2 for the case of twelve pump wavelengths. As can be seen in FIG. 2, the pump power decreases for increasing wavelength. Also, the spacing between wavelengths is closer for shorter wavelengths. FIG. 3 illustrates a relatively flat and broad Raman gain profile for a pump power-pump wavelength scheme similar to that of FIG. 2. The variations on the gain spectrum result in channel-to-channel variation in the optical-signal-to-noise-ratio (OSNR) and absolute signal power. Because system performance is limited by the OSNR of the worst performing wavelength, a large variation can severely limit system length. The maximum difference of the gain within the spectral range of signals is called gain ripple. The gain ripple of an amplifier should be as small as possible. This can be achieved by properly selecting the pump wavelengths and powers of the Raman amplifier. As can be seen in FIG. 3, the gain ripple over the wavelength range of 1520 to 1620 nm is smaller than 1.5 dB.
FIG. 4 is a schematic of a typical optical communication system using Raman amplifiers for periodic amplification of the optical signal. The system includes transmitter terminal 10 and receiver terminal 12. The transmitter terminal includes a number of optical communication transmitters 14a, 14b, . . . 14z respectively transmitting signals at optical communications wavelengths xcexa, xcexb, . . . xcexz.
The optical signals are multiplexed by multiplexer 16 and are amplified by a series of amplifiers A1, A2, . . . An. The signals are transmitted from the transmitter 10 to the amplifiers, between the amplifiers, and from the amplifiers to the receiver 12 via transmission optical fiber 26. For distributed Raman amplification, the optical amplifier will also include transmission optical fiber. The optical signals are then demultiplexed by demultiplexer 18 of receiver 12 to respective optical communications receivers 20a, 20b, . . . 20z. The demultiplexer 18 sends optical communications wavelengths xcexa, xcexb, . . . xcexz to respective optical communications receivers 20a, 20b, . . . 20z. 
Although FIG. 4 shows signals directed from transmitter terminal 10 to receiver terminal 12 for ease of illustration, in general the transmitter terminal 10 and receiver terminal 12 are typically transmitter/receiver terminals for bidirectional communication. In this case each of the transmitter/receiver terminals will have transmitters as well as receivers and both a multiplexer and demultiplexer.
FIG. 5 is a schematic of a typical distributed Raman optical amplifier 50 employed as one of the amplifiers in the series of amplifiers A1, A2, . . . An in the system of FIG. 4. The amplifier 50 includes optical pump assembly 51 (shown enclosed by dashed lines) and transmission fiber 64. In this amplification scheme, the pump assembly 51 includes a pump radiation source 52 that provides, for example, twelve different pump wavelengths xcex1 through xcex12. Specifically, the pump radiation source 52 comprises twelve lasers 56 that each emit radiation at a different wavelength of the wavelengths xcex1 through xcex12. The radiation from the individual radiation sources 56 of the pump radiation source 52 are then coupled or combined at pump radiation combiner 54, and the coupled radiation is output at pump radiation combiner output 58.
The coupled radiation has a coupled radiation profile that is a combination of the individual radiation profiles of the radiation input into the pump radiation combiner 54. The pump radiation profile, that will be coupled with the optical signal to be amplified, is therefore the coupled radiation profile in this case. Thus, the pump radiation profile is output from output 58. The pump radiation profile from output 58 is then coupled at pump-signal combiner 60 with the optical signal 62. Optical signal 62, i.e., the data signal, propagates in the transmission optical fiber 64 in a direction opposite to the radiation of the pump radiation profile. The optical signal is amplified along transmission optical fiber 62. Thus, the amplifier 50 and pump assembly 51 provide amplification for a single optical transmission path.
It would be desirable to provide an optical amplifier, such as a Raman amplifier, including a pump assembly that could amplify optical signals along several optical transmission paths. It would further be desirable to amplify along several optical transmission paths with the same set of radiation sources, such as lasers. It would further be desirable to provide an optical amplifier including a pump assembly that could allow for increased pump source redundancy without increasing the number of pump sources per optical transmission path amplified. In certain implementations it would be desirable to provide an optical amplifier including a pump assembly that would reduce gain ripple by providing an increased number of pump wavelengths.
According to one embodiment of the invention there is provided a pump assembly for an optical amplifier. The pump assembly comprises a plurality of pump radiation sources, each pump radiation source adapted to produce radiation having a set of pump wavelengths and pump powers corresponding to the respective pump wavelengths, wherein at least one set is different from at least one other set; a plurality of pump radiation combiners, each pump radiation combiner coupling the radiation of a respective set of pump wavelengths of a respective source of the plurality of pump radiation sources and outputting the respective coupled radiation having coupled radiation profiles via a respective pump radiation combiner output; a Pxc3x97P coupler, optically coupled to the outputs of the pump radiation combiners, that receives the coupled radiation from the pump radiation combiners and outputs P pump radiation profiles to respective of P coupler outputs; and a plurality of pump-signal combiners, each optically coupled to a respective coupler output of the P coupler outputs, which are adapted to couple an optical signal with the respective pump radiation profiles.
According to another embodiment of the invention there is provided an optical system. The optical system comprises an optical signal transmitter adapted to transmit multiple optical signals, the multiple optical signals having respective different wavelengths; an optical amplifier adapted to amplify at least one of the multiple optical signals; and an optical signal receiver adapted to receive the multiple optical signals including the amplified at least one of the multiple optical signals. The optical amplifier comprises a plurality of pump radiation sources, each pump radiation source adapted to produce radiation having a set of pump wavelengths and pump powers corresponding to the respective pump wavelengths, wherein at least one set is different from at least one other set; a plurality of pump radiation combiners, each pump radiation combiner coupling the radiation of a respective set of pump wavelengths of a respective source of the plurality of pump radiation sources and outputting the respective coupled radiation having coupled radiation profiles via a respective pump radiation combiner output; a Pxc3x97V coupler, optically coupled to the outputs of the pump radiation combiners, that receives the coupled radiation from the pump radiation combiners and outputs V pump radiation profiles to respective of V coupler outputs; and a plurality of pump-signal combiners, each optically coupled to a respective coupler output of the V coupler outputs, which are adapted to couple an optical signal with the respective pump radiation profiles.
According to another embodiment of the invention there is provided a method of amplifying optical signals. The method comprises providing a plurality of input radiation signals into the inputs of a Pxc3x97V coupler, each of the input radiation signals having a respective input radiation profile having a set of pump wavelengths and pump powers corresponding to the respective wavelengths, wherein at least one set is different from at least one other set; providing V output pump radiation profiles from the outputs of the Pxc3x97V coupler; and amplifying an optical data signal by coupling at least one of the V output pump radiation profiles with the optical data signal.
According to another embodiment of the invention there is provided a pump assembly for an optical amplifier. The pump assembly comprises a plurality of pump radiation sources, each pump radiation source adapted to produce radiation having a set of pump wavelengths and pump powers corresponding to the respective pump wavelengths, wherein at least one set is different from at least one other set; a plurality of pump radiation combiners, each pump radiation combiner coupling the radiation of a respective set of pump wavelengths of a respective source of the plurality of pump radiation sources and outputting the respective coupled radiation having coupled radiation profiles via a respective pump radiation combiner output; a Pxc3x97V coupler, optically coupled to the outputs of the pump radiation combiners, that receives the coupled radiation from the pump radiation combiners and outputs V pump radiation profiles to respective of V coupler outputs; and a plurality of V pump-signal combiners, each optically coupled to a respective coupler output of the V coupler outputs, which are adapted to couple an optical signal with the respective pump radiation profiles.
According to another aspect of the invention, and depending on the radiation source redundancy desired for the optical amplifier or pump assembly, all, most, some, or none of the wavelengths of the sets of wavelengths may be the same or adjacent to other wavelengths of the sets of wavelengths.